JACKSON, Wyo. (AP) — Investigators believe an improperly rigged belay device likely caused the 300-foot fall that killed a New York professor on a popular Grand Teton National Park climbing route in northwest Wyoming.

Park rangers have not finished their investigative report into the July 22 death of 33-year-old Marco Dees, of New Paltz, New York and the Netherlands. They'll never know with 100 percent certainty what happened.

But ranger Ryan Schuster says signs point toward Dees having made a rappelling mistake in rigging that resulted in a loose strand of rope pulling out of the belay device and through the anchor, causing Dees to fall.

Schuster says this type of rigging mistake is elementary but can happen to anyone.

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